The present invention relates to a disc-driving spindle motor in the OA/AV (Office Automation/Audio-Visual) field, particularly, for an audio/video apparatus using a digital disc/disk medium accommodated in a cartridge or a computer memory device.
A Audio-visual apparatuses and computer memory devices have been developed rapidly to be compact and denser in recent years. Among the technological innovations of this development, especially miniaturization, noise reduction, low power consumption, and long service life are required for a spindle motor rotating a disc.
With a compact spindle motor, the space assigned to a driving magnet or a stator coil determining the magnetic characteristics of the spindle motor is naturally limited, which might cause deterioration of the characteristics of the spindle motor. Meanwhile, a bearing of the motor is an important factor to determine the performance of the spindle motor.
A hydro-dynamic bearing has been noted and employed as a bearing which can satisfactorily cope with the above need. The hydro-dynamic bearing is constructed by a cylindrical rotary shaft and a hollow cylindrical sleeve metal fitted via a gap with the rotary shaft. A fluid (oil in many cases) is filled in the gap through herringbone grooves or the like formed in either of the rotary shaft and sleeve metal (sleeve bearing). The bearing supports a rotor by pressure generated in the fluid in accordance with the rotation of the rotor. This type of structure of the bearing is superior for a bearing of a spindle motor in principle in that the mechanism occupies little volume, noises during the rotation are lessened because of the arrangement of the rotor being supported via the fluid, shock resistance is high, and axial vibrations are reduced owing to an integrated effect where the load is supported at the whole periphery of the rotary shaft.
A conventional spindle motor will be described below with reference to drawing FIG. 3.
FIG. 3 is a sectional view of a conventional example. In FIG. 3, 1 is a magneto-optic disc medium (referred to as "a disc" hereinafter). 2 is a mounting plate formed of a ferromagnetic material and set in the disc 1. 16 is a spindle hub for receiving the disc 1. The spindle hub 16 has a clamp magnet 6 attached thereto which works to fix the disc 1 to the spindle hub 16 by a magnetic attraction force.
A rotary shaft 17 is installed at the center of the spindle hub 16, which is rotated together with the spindle hub 16 and the disc 1, thus serving as a rotary center for the spindle hub 16 and the disc 1. The rotary shaft 17 is supported by two ball bearings 18, 19.
A rotary driving force for the motor is generated by a rotary magnetic field formed by a stator core 11, having a stator coil 12 wound therearound when electricity is supplied to the stator core 11 to excite the stator core 11, and by a driving magnet 10 surrounding the periphery of the stator core 11 and magnetized with many polarities. A stator coil 12 is wound around the stator core 11. The driving magnet 10 is fixed to an inner peripheral surface of a frame 20, and the frame 20 has an upper end part thereof bent inward in FIG. 3 and fixed to the spindle hub 16. A rotor is constituted by the driving magnet 10 and the frame 20. The disc 1 is rotated by this rotor via the spindle hub 16. Additionally, the stator core 11 is secured to a bracket 7 and the stator coil 12 is wired to a printed board 9, thereby working as a driving force source.
In the above conventional constitution, far from satisfying the need for miniaturization, etc. described above, an issue that must be solved first is that there is a possibility that lubricating oil of the rotary shaft adheres to the disc because the disc is directly fitted with the rotary shaft.